1. Field of the Invention
The present invention relates to an image sampling method and an image processing method thereof, and more particularly, to a method that samples an image by serrated scanning in alternate rows and interlaced scanning.
2. Description of Related Art
An image sensor is used to translate an optical image focused on the sensor into electrical signals. One very well known type of the image sensor is the charge-coupled device (CCD). An integrated circuit chips containing CCDs are expensive since the specific manufacture process of CCD is required. In addition, the CCD also consumes large power dissipation. Recently, a active-pixel sensor (APS) produced by CMOS process has attracted much attention since it has capability of integrating the circuits, such as control circuit, driving circuit, and signal processing circuit, into a single sensor chip. The advantages of the CMOS APS are low power consumption, process compatibility with on-chip electronics and lower cost as compared with the CCD.
However, the image quality of the CMOS image sensor suffers from noises which will degrade the performance. These noises include thermal noise (i.e. KTC noise), 1/f noise and fixed pattern noise. The thermal noise is associated with the sampling of the image data, the 1/f noise is associated with the circuit which is used to amplify the image signal and the fixed pattern noise is associated with non-uniformity between columns within a pixel array. These noises become major factors causing the CMOS APS to have lower sensitivity or lower dynamic range of sampling as compared with the CCD. One of the ways to improve the sensitivity or dynamic range is to increase the exposure time as much as possible with a fixed frame rates.
FIG. 1 is a diagram of an image sensor. Referring to FIG. 1, the image sensor 100 includes a plurality of pixel element 110 arranged in rows and columns. The pixel element 110 includes a photodiode Pd and the transistors T1 through T3. It is assumed that the interlaced scanning system conforms to the standard specified by the national television standards committee (NTSC) so that an image is divided into an odd field and an even field to display; and the frame rate of the interlaced scanning system is 1/60 seconds per field. For convenience of description, the coordinates of the pixel element 110 is signed as (X, Y) and the pixel element 110 in (X1, Y1) is taken as an example. Referring to FIG. 1, the node N1 of the photodiode Pd is initially reset to a reference voltage Vref in the control of the transistor T1, which is turned on when the row line RST1 is active. After sufficient exposure time, the control line RD1 is active to turn on the transistor T3 so that the photodiode voltage at node N1 translated through the source follower transistor T2 can be read out via the column line RT1. Then, the photodiode voltage will be sampled and held in a following correlated doubled sampling (CDS) circuit (not illustrated). In accordance with the said principle, the photodiodes in each row are exposed to generate the voltage signal to the corresponding column lines in response to the state of the row line as well as the state of the corresponding control line.
With regard to the image sensor 100, a typical Bayer pattern color filter arrangement is deployed on the pixel elements 110 so that each pixel element 110 only senses one of either red or green or blue image information. In order to display a full color image of each pixel, an arithmetic calculation called color interpolation need to be employed on a pixel matrix. FIG. 2 is a diagram of a pixel array sampled from the image sensor 100 in FIG. 1. Referring to FIG. 2, taking the 3×3 pixel matrix 210a as an example, three consecutive row data is needed to implement color interpolation. For convenience of description, the image data of pixel in (X, Y) is signed as red data (R), green data (G) or blue data (B), wherein image data is obtained by sampling the image information. As the pixel matrix 210a shown, the pixel data in the intersections of three rows X1 through X3 and three columns Y1 through Y3 are utilized to interpolate the pixel in (X2, Y2). For example, the red data of the pixel in (X2, Y2) is an average of the red data of the pixels in (X1, Y2) and (X3, Y2). The green data of the pixel in (X2, Y2) is an average of the green data of the pixels in (X1, Y1), (X1, Y3), (X2, Y2), (X3, Y1) and (X3, Y3). The blue data of the pixel in (X2, Y2) is an average of blue data of the pixels in (X2, Y1) and (X2, Y3).
Referring to FIG. 1 and FIG. 2, by raster scanning and interlaced scanning, the pixel elements 110 are scanned and sampled in an order of rows X1, X3, X5, . . . X511 during an odd field period and the corresponding image data can be obtained to display the odd field. Next, the pixel elements 110 are scanned and sampled in an order of rows X2, X4, X6, . . . X512 during an even field period and the corresponding image data can be obtained to display the even field. Only odd rows are scanned during the odd field period; while only even rows are scanned during the even field period. It is obviously that an additional frame buffer occupying a significant cost is needed to store the odd field data or the even field data to implement color interpolation and further, the color interpolation can not be performed in real time since it needs at least three consecutive row data.
Referring to FIG. 1, as for the interlaced scanning system, the maximum exposure time of the pixel element 110 is 1/30 seconds since the photodiodes Pd in each odd row can be exposed from the beginning of the odd field period to the beginning of the next odd field period. If a progressive scanning is employed in the interlaced scanning system, the pixel elements 110 are sampled in an order of X1, X2, X3, . . . X512 during both of the odd field period and the even field period. As a result, not only it needs at least three line buffers to store pixel data for color interpolation, but also the exposure time of each pixel element is reduced by a half and thus the signal to noise ratio of the image is degraded under dark environment. How to enhance the image quality of the pixel element by maintain the maximum exposure time and eliminate an additional cost of the frame buffer for the interlaced scanning system becomes an important issue for CMOS sensor chip designer to improve.